Potassium dispersed on spinel oxides as catalysts for direct nox decomposition

ABSTRACT

Active catalysts for the treatment of a low temperature exhaust gas stream are provided containing potassium (K) dispersed on a spinel oxide for the direct, lean removal of nitrogen oxides from the exhaust gas stream. The low temperature (from about 400° C. to about 650° C.), direct decomposition is accomplished without the need of a reductant molecule. In one example, K may be dispersed on a surface of a metal oxide support, such as NiFe2O4 spinel oxide, synthesized using wet impregnation techniques. The K/NiFe2O4 catalyst system converts nitric oxide to nitrogen gas with high product specificity, up to 100%, avoiding the production of a significant concentration of the undesirable N2O product.

TECHNICAL FIELD

The present disclosure generally relates to catalysts for treatment ofan exhaust gas stream and, more particularly, to catalysts containingpotassium on a spinel for removal of nitrogen oxides from an exhaust gasstream as generated by an internal combustion engine, or the like.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it may be described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presenttechnology.

Catalysts effective at removing NOx from exhaust emissions are desirablein order to protect the environment and to comport with regulationsdirected to that purpose. It is preferable that such catalysts convertNOx to inert nitrogen gas, instead of converting NOx to othernitrogen-containing compounds. Catalysts that are effective at lowtemperature may have additional utility for vehicles.

Increasingly stringent NOx emission and fuel economy requirements forvehicles and automobile engines will require catalytic NOx abatementtechnologies that are effective under lean-burn conditions. Direct NOxdecomposition to N₂ and O₂ is an attractive alternative to NOx traps andselective catalytic reduction (SCR) for this application, as NOx trapsand SCR processes are highly dependent on reductants (such as unburnedhydrocarbons or CO) to mitigate NOx. The development of an effectivecatalyst for direct NOx decomposition would eliminate the use ofreducing agents, simplifying the NOx removal process, and thereforedecreasing the fuel efficiency cost of NOx abatement.

However, most catalysts active for direct NOx decomposition are onlyefficient at high temperatures (i.e., greater than about 600° C.), whichseverely limits a practical application for a vehicle exhaust gasstream. The most well-known low temperature (i.e., less than about 500°C.) direct NOx decomposition catalysts include Cu-ZSM5, K/Co₃O₄,Na/Co₃O₄, CuO, and Ag/Co₃O₄. However, low temperature activity andselectivity to N₂ for all of these catalysts is not sufficient forpractical application, and more advancements are needed. Advancements indirect NOx decomposition catalysis based on structure activityrelationships are lacking, and methodology to improve the performance ofspecific catalyst systems is needed.

Accordingly, it would be desirable to provide a catalyst for the removalof NOx from exhaust gas, that is effective at low temperature and thathas high N₂ product specificity.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In various aspects, the present teachings provide a catalyst system forthe direct decomposition removal of NO, from an exhaust gas stream. Incertain preferred aspects, the exhaust gas stream may be provided at atemperature of from about 400° C. to about 650° C. The catalyst systemmay include NiFe₂O₄ spinel oxide, and an alkali metal, such aspotassium, dispersed on a surface of the NiFe₂O₄ spinel oxide. Thecatalyst system is configured to catalyze decompose NOx to generate N₂without the presence of a reductant. The potassium may be provided in anamount of from about 0.5 wt % to about 1.5 wt %, or from about 0.5 wt %to about 1.0 wt % of the catalyst system, or in an amount of about 0.9wt % of the catalyst system.

In other aspects, the present teachings provide a catalytic converterfor the direct decomposition removal of NO, from an exhaust gas stream.The exhaust gas stream may be provided flowing through the catalyticconverter at a temperature of from about 400° C. to about 650° C. Thecatalytic converter may include an inlet configured to receive theexhaust gas stream into an enclosure, and an outlet configured to allowthe exhaust gas stream to exit the enclosure. A catalyst system may becontained inside the enclosure, the catalyst system including potassiumdispersed on a NiFe₂O₄ metal oxide, configured to catalyze a reductionof the NOx to generate N₂ without the presence of a reductant.

In still further aspects, the present teachings provide methods for thedirect decomposition removal of NO, from a low temperature exhaust gasstream. The methods may include flowing the exhaust gas stream through acatalyst system. This includes exposing the exhaust gas stream to acatalyst including potassium dispersed on a surface of a NiFe₂O₄ metaloxide support. The exposure results in catalyzing a reduction of the NOxto generate N₂ without the presence of a reductant. In various aspects,potassium is provided in an amount of about 0.9 wt % of the catalystsystem. Flowing the exhaust gas stream through the catalyst system at atemperature at or greater than about 450° C. may result in obtaining anNOx selectivity to N₂ greater than about 95%, and up to 100%.

Further areas of applicability and various methods of enhancing theabove coupling technology will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teachings will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 illustrates NO conversion profiles (activity) over NiFe₂O₄ andK/NiFe₂O₄ catalysts with respect to weight percentage of potassiumloading at various reaction temperatures;

FIG. 2 illustrates N₂ selectivity profiles over NiFe₂O₄ and K/NiFe₂O₄catalysts with respect to a weight percentage of potassium loading;

FIG. 3 illustrates x-ray diffraction profiles of the NiFe₂O₄ andK/NiFe₂O₄ catalysts at various weight percentages after calcination;

FIG. 4 illustrates x-ray diffraction profiles of the NiFe₂O₄ andK/NiFe₂O₄ catalysts at various weight percentages after the direct NOxdecomposition;

FIG. 5 illustrates in-situ FT-IR spectra for NiFe₂O₄ and K/NiFe₂O₄catalysts at various weight percentages during the NOx adsorption at300° C.; and

FIG. 6 illustrates O₂-TPD profiles of the NiFe₂O₄ and K/NiFe₂O₄catalysts at various weight percentages.

It should be noted that the figures set forth herein are intended toexemplify the general characteristics of the methods, algorithms, anddevices among those of the present technology, for the purpose of thedescription of certain aspects. These figures may not precisely reflectthe characteristics of any given aspect, and are not necessarilyintended to define or limit specific embodiments within the scope ofthis technology. Further, certain aspects may incorporate features froma combination of figures.

DETAILED DESCRIPTION

The present teachings provide an active catalyst system for thetreatment of a low temperature exhaust gas stream. The catalyst systemincludes the dispersion or impregnation of potassium (K), an alkalimetal, onto a surface of a NiFe₂O₄ metal oxide, such as a spinel, forthe direct, lean removal of nitrogen oxides from the exhaust gas stream.This catalyst system has shown to vastly improve the direct NOx activityas well as N₂ selectivity. The direct NOx activity of NiFe₂O₄ increaseswith increasing K loading up to an optimum of 0.9 wt %. Further increasein the K loading leads to a decrease in the direct NOx activity.Importantly, the selectivity towards N₂ increases with increasing Kloading, and 1.5 wt % K/NiFe₂O₄ exhibited 100% N₂ selectivity at 450° C.X-ray powder diffraction measurements show that K was well dispersed onthe surface of NiFe₂O₄ and does not significantly affect the NiFe₂O₄structure during calcination or under direct NOx decomposition reactionconditions. In-situ FT-IR measurements during NOx adsorption at 300° C.show that impregnation of K on NiFe₂O₄ creates new NOx adsorption siteswhich yield very active nitrite intermediates that lead to the increasedthe direct NOx activity. O₂-TPD measurements show that impregnation of0.5 wt % and 0.9 wt % K on the NiFe₂O₄ surface do not significantlyalter the oxygen release characteristics of the NiFe₂O₄. However, at 1.5wt % K, the bulk oxygen release shifts to higher temperature which isresponsible for the decrease in activity compared to the 0.9 wt % K.These results show that direct NOx activity is optimized over catalystswith both lower temperature oxygen release characteristics and NOxadsorption sites that yield surface nitrites species.

The low temperature (i.e., from about 400° C. to about 650° C.), directdecomposition is accomplished without the need of a reductant (i.e., H₂,CO, C₃H₆ or other hydrocarbons, and/or soot), thereby improving fuelefficiency. Direct decomposition, as discussed herein, refers tocatalytic transformation of nitrogen oxides to elemental nitrogen andoxygen. This differs, for example, from catalytic reduction of nitrogenoxides to ammonia and water. In one example, K may be dispersed orsubstantially uniformly spread out on a surface of a metal oxidesupport, such as the NiFe₂O₄ spinel oxide, synthesized using a wetimpregnation technique. The K/NiFe₂O₄ catalyst system converts nitricoxide to nitrogen gas with high product specificity, all while avoidingthe production of a significant concentration of the undesirable N₂Oproduct. In various preferred aspects, the K/NiFe₂O₄ catalyst may beoperable at exhaust gas/stream temperatures lower than about 650° C.,lower than about 600° C., lower than about 550° C., lower than about500° C., lower than about 450° C., and even lower than or at about 400°C.

The presently disclosed catalyst system includes methods for dispersingpotassium on a metal oxide support, specifically a spinel oxide (i.e.,NiFe₂O₄), via wet impregnation techniques. This method particularlyprovides for improved total yield of product N₂ and product selectivityto N₂ (versus undesired N₂O and/or NO₂ products) during low temperaturedirect NOx decomposition as compared to either the bare NiFe₂O₄ spineloxide support only. Because of the high selectivity to N₂ for thepresent teachings, the undesirable N₂O product is not produced in asignificant quantity during the direct NO decomposition over NiFe₂O₄spinel-supported potassium. Additionally, it has been discovered that,on a wt % basis, the addition of about 0.9 wt % K to the surface of theNiFe₂O₄ spinel improves the selectivity to the N₂ product from 20%(without K) to greater than about 85% at a temperature of about 450° C.,and greater than about 95% at a temperature of about 500° C. Theaddition of about 1.5 wt % K to the surface of the NiFe₂O₄ spinelimproves the selectivity to about 100% at all temperatures of from about450° C. to about 650° C.

As detailed herein, the present teachings not only include thedevelopment of the catalyst system, but also the utilization of thecatalyst system with exhaust gas streams, particularly with catalyticconverters for vehicles, automobiles, and the like, as well as includingmethods of synthesizing the potassium supported in the spinel oxide.

Furthermore, the activity of the spinel supported potassium can beoptimized by different loadings, or the amount of potassium present inthe catalyst system by weight. For example, in various preferredaspects, the potassium is present in amount of from about 0.5 wt % toabout 1.5 wt % of the catalyst, or from about 0.5 wt % to about 1 wt %,or in an amount of about 0.9 wt %. In one specific example, the additionof potassium in the catalyst system improves the activity by abouttwenty (20) times at about 650° C. as compared to a NiFe₂O₄ catalystwithout the presence of potassium.

The catalyst systems of the present disclosure can be used in a chamberor an enclosure, such as a catalytic converter, having an inlet and anoutlet. As is commonly known to those of ordinary skill in the art, sucha chamber or enclosure can be configured to receive an exhaust gasstream through the inlet and to exit the exhaust gas stream through theoutlet, such that the exhaust gas stream has a particular or definedflow direction.

EXAMPLES

Various aspects of the present disclosure are further illustrated withrespect to the following Examples. It is to be understood that theseExamples are provided to illustrate specific embodiments of the presentdisclosure and should not be construed as limiting the scope of thepresent disclosure in or to any particular aspect.

Synthesis and Material Characterization

The NiFe₂O₄ may be purchased from a commercial supplier, such as SigmaAldrich, and calcined at about 400° C. for about 1 hour. In variousaspects, the NiFe₂O₄ may be in a nanoparticle form, having an averagediameter of from about 2 nm to about 100 nm.

The K/NiFe₂O₄ catalyst systems according to the present technology maybe synthesized by using a wet impregnation method. In one exemplarysynthesis procedure, 5 g of NiFe₂O₄ can be mixed with 50 mL of water.Next, the required quantity of potassium hydroxide can be dissolvedseparately in deionized water and combined with the NiFe₂O₄ suspension.The mixture may then be heated to about 80° C. with continuous stirring.The resulting powder can be dried in an oven at about 120° C. for about12 h under air. Finally, the catalyst system can be calcined at about400° C. for about 1 h in the presence of air with a 1° C./min ramp.Different loadings of K, for example, 0.5 wt %, 0.9 wt %, and 1.5 wt %on NiFe₂O₄ can be synthesized using a similar procedure by changing theamount of potassium hydroxide precursor during the synthesis as isconventionally done.

The phase composition of spinels can be measured using X-ray diffractionmeasurements. In one example, X-ray powder diffraction (XRD)measurements can be performed using a Rigaku SmartLab X-RayDiffractometer. Spectra can be collected over a 2θ range of betweenabout 20 to about 80 degrees, at a rate of about 0.5 deg./min, and witha step size of about 0.02 deg./step. Structural assignments can then bemade using PDXL software. The phase composition of the materials can bedetermined using an ICDD-PDF database.

Oxygen release characteristics of the NiFe₂O₄ and K/NiFe₂O₄ catalystscan be studied using O₂ temperature programmed desorption (O₂ TPD)experiments. In one example, O₂ TPD experiments can be performed using aNETZSCH STA-449 thermogravimetric analyzer equipped with massspectrometer. Before the experiment, the catalysts may be preheated toabout 300° C. in the presence of 20% O₂/He. After the pretreatment, thetemperature can be decreased to about 100° C. Oxygen releasecharacteristics can be studied by heating the catalyst from betweenabout 100° C. to about 600° C. in the presence of helium. The oxygensignal can be monitored using mass spectrometry. The O₂ TPD profiles canbe presented with temperature as a function of the amount of oxygenreleased. Typically, behavior for the catalysts can be as follows:physisorbed oxygen releases below about 200° C., chemisorbed oxygenreleases from between about 200° C. to about 450° C., and finally thebulk oxygen releases after about 450° C.

The NO adsorption properties can be measured using in situ Fouriertransform infrared (FT-IR) spectroscopic measurements. In one example, aHarrick High Temperature Cell with environmental (gas flow) andtemperature control can be used for in situ diffuse-reflectance FT-IRspectroscopy. Spectra can be recorded using a Thermo Scientific Nicolet8700 Research FT-IR Spectrometer equipped with a liquid N₂ cooled MCTdetector. Spectra can be obtained with a resolution of 2 cm′ and byaveraging 64 scans. In situ diffuse-reflectance FT-IR spectra can becollected during NO adsorption at about 300° C. Prior to NO adsorption,the sample can first be pretreated at about 350° C. in 30 ml/min of 10%O₂/He. The background spectrum (64 scans) is of the catalyst aftercooling to about 300° C. in 30 ml/min of UHP He. Adsorption of NO can beachieved by flowing 30 ml/min of 1% NO over the catalyst for about 25min. Adsorption of NO can be allowed to proceed for about 25 min whilespectra is obtained every minute using a series collection. To comparepeak intensities among different catalyst samples, the adsorptionspectra can be normalized to the NO gas phase peak at about 1876/cm.

The direct NOx decomposition measurements for the present technology maybe performed in a fixed bed flow reactor following a predeterminedscheme. For example, a pretreatment step may begin with catalysts beingpretreated at a temperature of about 500° C. in the presence of 20%O₂/He. After the pretreatment, the bed temperature is decreased to about350° C. before direct NOx decomposition measurements are collected. Thedirect NOx decomposition measurements are performed using about 1%NO_(x) balance helium with a gas hourly space velocity of 2,100/h and inthe temperature regions of about 350° C.-650° C. For example, thetemperature is held at 350° C. for about 2 hours, raised to 450° C. forabout two hours, continuing up to 550° C. for about two hours, and thenup to about 650° C. for about two hours.

Performance Evaluation

For direct NOx decomposition to occur, NO must directly decompose to N₂and O₂ over the catalyst surface. However, there is a possibility forunwanted N₂O and NO₂ formation as side products. Therefore, in additionto high NO conversion, it is also very important to have higherselectivity towards N₂+O₂ formation rather than N₂O or NO₂. The reactioncan be represented as:

(4a+4c−2b)NO→aN₂ +bO₂ +cN₂O+(2a−2b+c)NO₂

In this regard, the selectivity towards N₂ can be defined as:

N₂ selectivity (%)=2*[N₂]/(2*[N₂]+0.5[N₂O]+[NO₂])

For the performance evaluation considerations, the catalyst systems ofthe present technology are first calcined at about 400° C. for about 1hour. After being calcined, direct NOx decomposition is performed overNiFe₂O₄ and various K/NiFe₂O₄ catalysts.

FIG. 1 illustrates the NOx activity of the NiFe₂O₄ and various K/NiFe₂O₄catalyst systems as a function of potassium weight percent loading forvarious reaction temperatures. As shown in FIG. 1, the NOx decompositionactivity increases with increasing temperature from 450° C. to 650° C.for all of the wt % K loadings. As clearly shown, the addition ofpotassium to the NiFe₂O₄ spinel improves the direct NOx decompositionactivity at all temperatures. The activity increases with increasingpotassium loading, up to an optimum loading of 0.9 wt % K. Furtherincreases in the potassium loading leads to a slight decrease in theactivity. As shown, the 0.9 wt % K/NiFe₂O₄ exhibits an activity greaterthan about 0.07 μmol/g/s, which is about twenty (20) times higher at650° C. as compared to the activity of the NiFe₂O₄ catalyst withoutpotassium.

To confirm direct NOx decomposition to N₂ is taking place, rather thanthe unwanted side products of N₂O or NO₂, the N₂ selectivity may becalculated. Using the FTIR detector for product analysis, it is possibleto detect NO, N₂O, and NO₂ species from the outlet of the reactor duringdirect NOx decomposition evaluation. FIG. 2 illustrates the N₂selectivity profiles calculated for the NiFe₂O₄ and K/NiFe₂O₄ spinelsfrom 450° C. to 650° C. As shown in FIG. 2, the N₂ selectivity increaseswith increasing potassium loading on NiFe₂O₄. Remarkably, addition of1.5 wt % K to the NiFe₂O₄ improves the N₂ selectivity from about 17%(without potassium) to 100% at 450° C. Additionally, as shown, all ofthe catalyst systems exhibit nearly 100% N₂ selectivity as tested in atemperature range from between about 500° C. to 650° C. These resultssuggest that K/NiFe₂O₄ catalysts have the potential as very goodcandidates for direct NOx decomposition because they exhibit very goodactivity and selectivity to N₂ at temperatures as low as 450° C.

The performance evaluation of the present technology also includesstructural and surface characterization measurements. For example, thesecharacterizations can be performed over NiFe₂O₄, and K/NiFe₂O₄ catalyststo better understand the influence of potassium deposition on theNiFe₂O₄ spinel. In this regard, FIG. 3 illustrates X-ray diffraction(XRD) profiles of NiFe₂O₄ and K/NiFe₂O₄ catalysts after calcination. Asshown, after calcination, NiFe₂O₄ exhibits peaks at 30.14, 35.94, 37.31,43.34, 53.86, 57.45, 62.93 degrees. These 2θ values correspond toreflections of (220), (311), (222), (400), (422), (511), and (440)planes that are indications of the presence of the cubic spinelstructure. These diffraction lines provide clear evidence of thepresence of NiFe₂O₄. For example, all of the diffraction peaks matchwell with the reported values (JCPDS file No: 10-325), and are indexedwith the lattice parameter of a=8.339±1° A. All of the potassium loadedK/NiFe₂O₄ catalyst systems exhibit reflections similar to NiFe₂O₄. Noadditional peaks are observed. Additionally, no shift in the peakpositions are observed. These results suggest that potassium is welldispersed over the surface of NiFe₂O₄ and did not incorporate into thespinel structure in a way that was significant enough to effect the longrange order of the system.

FIG. 4 illustrates the X-ray powder diffraction patterns of the NiFe₂O₄and K/NiFe₂O₄ catalysts after the direct NOx decomposition. The NiFe₂O₄exhibits peaks due to only spinel NiFe₂O₄ after the direct NOxdecomposition. No additional peaks are observed, for example, due toeither Fe₂O₃ or NiO. This finding does not explain the role of K, butsuggests another reason why NiFe₂O₄ is a very good candidate for directNOx decomposition, i.e. structural stability during reaction. All thepotassium impregnated NiFe₂O₄ catalyst systems exhibit similar peaks inthe X-ray diffraction pattern as NiFe₂O₄ after the direct NOxdecomposition. These results show that the potassium addition does havea detectable effect on the structure of NiFe₂O₄ during calcination orduring the direct NOx decomposition.

The NOx adsorption properties of NiFe₂O₄ and K/NiFe₂O₄ catalysts can beinvestigated using in situ FT-IR spectroscopic measurements. FIG. 5illustrates the in situ FT-IR spectra of NiFe₂O₄ and K/NiFe₂O₄ catalystsystems during NOx adsorption at 300° C. NiFe₂O₄ exhibits peaks at 1565,1257, and 1004 cm⁻¹ wavenumbers. These peaks are corresponding tobidentate or bridging nitrate intermediates. As shown in FIG. 5, thedeposition of 0.5 wt % on the NiFe₂O₄ exhibits additional peaks at 1480,1193, 1070 cm⁻¹ wavenumbers along with the nitrate peaks. These peaksare due to nitrite intermediates. Because of the broadness of the 1480cm⁻¹ peak, it may be difficult to make a detailed assignment anddifferentiate between bridging nitro-nitrito or mono-dentate nitritespecies. However, it is clear that NiFe₂O₄ forms nitrate intermediates,and potassium promotion leads to the nitrite intermediates. FIG. 5 showsthat the intensities of the peaks due to the nitrate species decreaseswith increasing potassium loading, and disappear completely for the 1.5wt % K/NiFe₂O₄ sample.

Prior studies have reported that the nitrate species formed on Ba/MgOare spectator species in the NO decomposition reaction and actuallypoison the reaction sites due to their strong adsorption. Prior studiesalso reported that nitrite intermediates are very active and readilydecompose to N₂ and O₂. With the present technology, deposition ofpotassium leads to the formation of nitrite intermediates and improvesthe direct NOx activity and selectivity of the NiFe₂O₄ catalyst. The 1.5wt % K/NiFe₂O₄ only forms nitrite intermediates, and for this reason,this catalyst exhibits 100% selectivity towards N₂ at 450° C.

The oxygen release characteristics of the NiFe₂O₄ and K/NiFe₂O₄ catalystsystems can be studied using O₂-TPD measurements. FIG. 6 illustrates theO₂-TPD profiles of the NiFe₂O₄ and K/NiFe₂O₄ catalyst systems. Oxygenrelease at lower temperature is one of the important factors that leadsto direct NOx decomposition activity because the release/desorption ofoxygen as molecular O₂ in a stoichiometric ratio to N₂ formation is arequirement of this reaction. The O₂-TPD profiles of FIG. 6 arepresented with temperature as a function of the amount of oxygenreleased. For the pure NiFe₂O₄ spinel, no physisorbed oxygen release isobserved. Chemisorbed oxygen released between 150° C.-350° C., and thebulk oxygen release started at 375° C. and finished at 590° C. Both 0.5wt % K/NiFe₂O₄ and 0.9 wt % K/NiFe₂O₄ catalyst exhibited a similarO₂-TPD profile to the NiFe₂O₄ spinel, and also there is not much changein the bulk oxygen release peak temperature. On the other hand, the 1.5wt % K/NiFe₂O₄ releases bulk oxygen at much higher temperature (note theannotated 80° C. shift in the peak) as compared to the NiFe₂O₄, 0.5 wt %K/NiFe₂O₄, and 0.9 wt % K/NiFe₂O₄ catalysts. These results suggest thatpotassium may inhibit the oxygen release of NiFe₂O₄ after 0.9 wt %deposition, and that this factor may be responsible for the decrease indirect NOx decomposition activity beyond the optimum loading of 0.9 wt %K/NiFe₂O₄ as discussed above and with renewed reference to FIG. 1.

The preceding description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. As usedherein, the phrase at least one of A, B, and C should be construed tomean a logical (A or B or C), using a non-exclusive logical “or.” Itshould be understood that the various steps within a method may beexecuted in different order without altering the principles of thepresent disclosure. Disclosure of ranges includes disclosure of allranges and subdivided ranges within the entire range.

The headings (such as “Background” and “Summary”) and sub-headings usedherein are intended only for general organization of topics within thepresent disclosure, and are not intended to limit the disclosure of thetechnology or any aspect thereof. The recitation of multiple embodimentshaving stated features is not intended to exclude other embodimentshaving additional features, or other embodiments incorporating differentcombinations of the stated features.

As used herein, the terms “comprise” and “include” and their variantsare intended to be non-limiting, such that recitation of items insuccession or a list is not to the exclusion of other like items thatmay also be useful in the devices and methods of this technology.Similarly, the terms “can” and “may” and their variants are intended tobe non-limiting, such that recitation that an embodiment can or maycomprise certain elements or features does not exclude other embodimentsof the present technology that do not contain those elements orfeatures.

The broad teachings of the present disclosure can be implemented in avariety of forms. Therefore, while this disclosure includes particularexamples, the true scope of the disclosure should not be so limitedsince other modifications will become apparent to the skilledpractitioner upon a study of the specification and the following claims.Reference herein to one aspect, or various aspects means that aparticular feature, structure, or characteristic described in connectionwith an embodiment or particular system is included in at least oneembodiment or aspect. The appearances of the phrase “in one aspect” (orvariations thereof) are not necessarily referring to the same aspect orembodiment. It should be also understood that the various method stepsdiscussed herein do not have to be carried out in the same order asdepicted, and not each method step is required in each aspect orembodiment.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations should not beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

1. A catalytic converter for the direct decomposition removal of NO_(x)from an exhaust gas stream flowing at a temperature of from about 400°C. to about 650° C., the catalytic converter comprising: an inletconfigured to receive the exhaust gas stream into an enclosure; anoutlet configured to allow the exhaust gas stream to exit the enclosure;and a catalyst system contained inside the enclosure, the catalystsystem comprising potassium (K) dispersed on a NiFe₂O₄ spinel oxide,configured to catalyze a reduction of the NOx to generate N₂ without thepresence of a reductant.
 2. (canceled)
 3. The catalytic converteraccording to claim 1, wherein the NiFe₂O₄ spinel oxide is in ananoparticle form, having an average diameter of from about 2 nm toabout 100 nm.
 4. The catalytic converter according to claim 1, whereinthe K is dispersed on a surface of the spinel oxide and provided in anamount of from about 0.5 wt % to about 1.0 wt % of the catalyst system.5. The catalytic converter according to claim 4, wherein the K isdispersed on a surface of the spinel oxide and provided in an amount ofabout 0.9 wt % of the catalyst system.
 6. The catalytic converteraccording to claim 5, configured to flow the exhaust gas stream throughthe catalyst system at a temperature at or greater than about 500° C.and obtaining an NOx selectivity to N₂ greater than about 95%.
 7. Thecatalytic converter according to claim 1, wherein the K is dispersed ona surface of the spinel oxide and provided in an amount of about 1.5 wt% of the catalyst system, and the catalytic converter is configured toflow the exhaust gas stream through the catalyst system at a temperatureat or greater than about 450° C. and obtaining an NOx selectivity to N₂of about 100%.
 8. A method for direct decomposition removal of NO, froman exhaust gas stream, the method comprising: flowing the exhaust gasstream through a catalytic converter and exposing the exhaust gas streamto a catalyst system comprising potassium (K) dispersed on a surface ofa NiFe₂O₄ spinel oxide; and catalyzing a reduction of the NOx togenerate N₂ without the presence of a reductant.
 9. (canceled)
 10. Themethod according to claim 8, comprising flowing the exhaust gas streamthrough the catalyst system at a temperature of from about 400° C. toabout 650° C.
 11. The method according to claim 8, wherein the K isprovided in an amount of from about 0.5 wt % to about 1.0 wt % of thecatalyst system.
 12. The method according to claim 8, wherein the K isprovided in an amount of from about 0.75 wt % to about 1.0 wt % of thecatalyst system.
 13. The method according to claim 8, wherein the K isprovided in an amount of about 0.9 wt % of the catalyst system.
 14. Themethod according to claim 13, comprising flowing the exhaust gas streamthrough the catalyst system at a temperature at or greater than about500° C. and obtaining an NOx selectivity to N₂ greater than about 95%.15. The method according to claim 8, wherein the K is dispersed on asurface of the spinel oxide and provided in an amount of about 1.5 wt %of the catalyst system, and the method comprises flowing the exhaust gasstream through the catalyst system at a temperature at or greater thanabout 450° C. and obtaining an NOx selectivity to N₂ of about 100%.